Hi Stefan,
You really should use the search function here, or search with Google, it will look in here and post results. This is a very old question that's been asked quite a few times. No intention to be rude but learning to use search here will benefit you greatly over the long run. The longer you have the Z the more you will want to know and some of it requires learning to use search on here.
To answer your original question, each car has a unique UUID *Unique Universal Identifier Code* and you must have that key to open up the Navi to updating, without that code *you buy that when you get the map update*, you cannot update the Navi files.
You can get the discs many locations on the "net" but the Navi won't accept them without the code.
Of course it's possible to hack it but then someone like that wouldn't be asking on this forum
.